(a) Field of the Invention
The present invention relates to a process for recovering trimethoxysilane and in particular to a process for recovering trimethoxysilane from a mixture of trimethoxysilane and methanol.
The present invention also relates to a continuous process for producing and purifying trimethoxysilane by first reacting silicon metal and methanol in the presence of a copper catalyst to produce a product which includes, along with trimethoxysilane, a trimethoxysilane-methanolazeotrope. Thereafter using tetramethoxysilane an extractive distillation is performed to recover trimethoxysilane from the mixture. The methanol separated from the tetramethoxysilane/trimethoxysilane mixture can be recycled for use in the reaction while the tetramethoxysilane can be reused as the extractive distillation solvent.
(b) Prior Art
The reaction between silicon metal and alcohol is well established. As long ago as 1949, U.S. Pat. No. 2,473,260 issued to Rochow described a process for the preparation of methyl silicates from methanol and silicon-copper masses. Similarly, U.S. Pat. No. 3,072,700 taught the preparation of silanes [(RO).sub.3 SiH, (RO).sub.2 SiH.sub.2 ] from silicon metal and alcohol in a fluidized bed reactor.
Patents on the similar production of tetraalkylorthosilicates include U.S. Pat. No. 4,288,604 and Japanese Pat. No. 1979-163529. Patents covering the similar production of trialkoxysilanes include U.S. Pat. No. 3,775,457. See also Japanese Pat. Nos. 1979-163529, 1980-28929, 1980-28928, 1980-2641, and Japanese laid-open application Nos. 33457/1980 and 11538/1980.
However any synthesis of trimethoxysilane which employs an excess amount of methanol is plagued by the loss of trimethoxysilane in an azeotrope with the excess methanol since trimethoxysilane and methanol form a minimum boiling azeotrope. Only that component of the trimethoxysilane and methanol mixture which is present in excess of the azeotropic proportion can be recovered by simple distillation techniques.
In Japanese Pat. Nos. 11538/1980 and 33457/1980 there is disclosed a process employing azeotropic distillations which have employed hexane in an attempt to form a second azeotrope with methanol which can then be boiled off to leave trimethoxysilane behind as an end product. This technique is not preferred since it introduces a new component to the system which may contaminate the end product and increases the cost of the final product. Also, this technique is difficult to control in a continuous mode, since it requires maintaining by continuous feed the exact ratio of hexane and methanol to form the lower boiling azeotrope between methanol and hexane. Finally, hexane and trimethoxysilane can form an azeotrope which will complicate the recovery of trimethoxysilane.
Accordingly, a need continues to exist for a commercially attractive process to recover trimethoxysilane from a trimethoxysilane-methanolazeotrope.